NXTalk: Smalltalk on Lego Mindstorms NXT

Application for the 2009 ESUG Innovation Technology Award
<strong>NXTalk</strong>: <strong>Smalltalk</strong> on Lego Mindstorms NXT
Application Details
Name of the Software: <strong>NXTalk</strong>
Names of Developers: Martin Beck, Michael Haupt, Robert Hirschfeld
Affiliations of Developers: Software Architecture Group
Hasso-Plattner-Institut
Prof.-Dr.-Helmert-Str. 2-3
D-14482 Potsdam
Germany
Primary Contact: michael.haupt@hpi.uni-potsdam.de
<strong>Smalltalk</strong> Dialect Used: Squeak, <strong>NXTalk</strong>
Project Web Page: www.hpi.uni-potsdam.de/swa/projects/nxtalk/index.html
Keywords
Lego Mindstorms NXT, robots, embedded <strong>Smalltalk</strong>, virtual machine
Licence Information
The <strong>NXTalk</strong> VM and development environment are licensed under the MIT
License. NXOS, which is the operating system in which <strong>NXTalk</strong> is running on
the NXT, is licensed under the GPL but not part of our contribution.
Description
<strong>NXTalk</strong> is an integrated approach to making <strong>Smalltalk</strong> programming available
on the Lego Mindstorms NXT [1] platform. It consists of two building blocks:
the <strong>NXTalk</strong> virtual machine (VM) running on the NXT hardware, and the
<strong>NXTalk</strong> programming environment, which is implemented in Squeak [2].
Figure 1: <strong>NXTalk</strong> Code in a Squeak Browser and Workspace

Programming <strong>NXTalk</strong>
An <strong>NXTalk</strong> program entirely looks and feels like an ordinary <strong>Smalltalk</strong>
program; the programming tools available in Squeak can be used seamlessly
to develop software for the NXT. To some degree, <strong>NXTalk</strong> programs can also
be simulated in Squeak.
<strong>NXTalk</strong> Standard Libraries
<strong>NXTalk</strong> provides its own standard libraries, which are designed with special
regards to the NXT's memory constraints. Thus, the class hierarchy is highly
condensed while still offering the typical <strong>Smalltalk</strong> classes and methods.
Additionally, special classes resembling the NXT's hardware devices like
sensors, motors or buttons exist, providing easy and <strong>Smalltalk</strong>-like access to
these entities. Although optimised for size, <strong>NXTalk</strong> has—besides libraries for
collections, string manipulation or event handling—a full meta-layer. Not only
is the cyclic meta-class structure accessible, but also each class’ method
dictionary.
<strong>NXTalk</strong> Applications and Deployment
Like most <strong>Smalltalk</strong> implementations, <strong>NXTalk</strong> is image-based. In order to
replace code or deploy applications, it uses <strong>Smalltalk</strong>’s meta-facilities. Thus,
the virtual machine merely provides functionality for loading, starting and
running an image, while the responsibilities for deployment and application
management lie in the image itself and are written in <strong>Smalltalk</strong>. An <strong>NXTalk</strong>
application or package consists of several compiled classes and additional
meta-information such as the application’s name. The compilation process
takes place on the Squeak side and the created packages along with its
bytecode and data are transferred to <strong>NXTalk</strong> via remote method invocation.
Nevertheless, the initial image has to be built up completely in Squeak and
uploaded to the NXT in the first place.
Figure 2: Line Follower in Action

The <strong>NXTalk</strong> Virtual Machine
The <strong>NXTalk</strong> virtual machine is a <strong>Smalltalk</strong> VM written in pure C and tightly
coupled to the hardware of the Lego Mindstorms NXT robot. It uses NXOS [3],
an open source operating system for the NXT, as its underlying base. Several
specialised algorithms had to be developed in order to overcome the
hardware limitations of the NXT. With 64 kB RAM and 256 kB Flash, the
memory constraints were the main subject in virtual machine design. Thus, a
very dense object layout was created, saving space for <strong>Smalltalk</strong>’s typical
many small objects. At the same time, these size optimisations had to be
taken with caution, as system performance has a great effect on user
experience. The <strong>NXTalk</strong> virtual machine tries to balance these contradictory
requirements in a reasonable fashion, while still providing a full <strong>Smalltalk</strong>
environment and open access to the NXT's hardware devices like USB or
Bluetooth.
Outlook
We are working on extending support of <strong>NXTalk</strong> for both the target audience
and with respect to supported hardware. Regarding the latter, more hardware
drivers—for other than the default sensors shipped with the NXT—have to be
implemented at the level of the virtual machine, and the according <strong>NXTalk</strong>
classes have to be provided at the <strong>Smalltalk</strong> level.
Regarding target audience, we are working on providing an Etoys interface for
<strong>NXTalk</strong>, making robot programming available at both the levels of scriptable
objects and actual hardware. This extension will allow for putting
programming experience gathered in, e.g., BotsInc to practice using real
robots.
References
[1] www.mindstorms.com
[2] www.squeak.org
[3] nxt.natulte.net/nxos/trac